In the given work the system “metal - filter cake” in application to well drilling is investigated on the basis of B.V.Derjaguin conception about the binomial law of friction. The appropriate method for study of antifriction properties of filter cake is offered. The influence of solid phase dispersion ability of a clay drilling fluid on its frictional properties is investigated.
There are different perspectives on the nature of occurrence of resistance forces to movement of drill strings in wells caused by quality of a drilling fluid. However majority of the researchers mark presence always, alongside with frictional forces, molecular forces playing a significant role at the drags on and sticking of drill string [1,2,3].
With the purpose of studying of the mechanism of friction at phases contact interaction, eliciting some basic physico-chemical factors and definition their quantitative estimation, in the given work a new approach studying of this process is considered, which is based on B.V.Derjaguin, A.D.Zimon and E.I.Andrianov works[4,5].
As is known, one of the laws of friction is the Amonton law according to which the resistance to shear force of is directly proportional to normal loading on a friction pair and does not depend on the area of contact:
However Amonton law is truth for systems, in which both contacting phases are represented by homogeneous continuous mediums. A filter cake of a drilling fluid is a high-concentrated dispersing system with thixotropy properties, structurally nonuniform on thickness, the filter cake density increases from the top layers to bottom. The numerous experimental researches at study of friction arising in pair "metal – filter cake" show, that the similar law is not observed. Actually the relationship of resistance to shear of force against normal load in the considered system essentially depends on the area of contact, therefore it is expedient to consider the relationship between specific forces s and Ns (forces are referred to unit of the contact area). The relationship s against Ns is not characterized by the straight line 1, and a straight line 2, which passes above the straight line 1 and is described by B.V.Derjaguin binomial law of friction : s=(Ns+Fs).In the given equation the value Fs is a resultant force of intermolecular interactions. In a case if the shear is directly on the contact boundary "metal – filter cake", the force Fs is a consequence of adhesion Fs(ad), if there is the direct shear inside filter cake, Fs is a consequence of autohesion Fs(aut). The friction factor is equal to the tangent of an angle φ of an inclination of a straight line to the abscissa axis and characterizes the frictional force interfering movement of filter cake particles on a surface of metal (at adhesion) or relative movement of particles inside filter cake (at autohesion).It is obvious, the direction of resistance forces in system "metal – filter cake" is possible by the variation of the adhesion, autohesion and friction factor values. From a fig. 1 it is visible, that the straight line 2 cuts off some segments on coordinate axes. The segment on the abscissa axis Fs(ad/aut) characterizes breaking strength of contact and the segment on the ordinate axis Cs(ad/aut) - shear strength of contact. The values Fs/aut) and Cs/aut) are a consequence of autohesion, if the shear plane is in the filter cake, and Fs(ad) and Cs(ad) – a consequence of adhesion, if the shear is directly on the contact boundary "metal – filter cake". In the contact of two bodies distinguished by strength, the shear is not on their contact boundary, and inside a more plastic body. If the shear is directly inside a filter cake, in the absence of load (Ns = 0), the frictional factor is determined by shear specific strength to breaking specific strength ratio for a filter cake: = Сs(aut)/Fs(aut).
MATERIALS AND METHODS
The following model fluids were the objects of research:
№1 - 10% kuganaksk gel material + 0,5% carboxymethyl cellulose + water;
№2 - 10% kuganaksk gel material + 0,5% carboxymethyl cellulose + water+ 0,125% Na2CO3;
№3 - 10% kuganaksk gel material + 0,5% carboxymethyl cellulose + water + 0,25% Na2CO3
In the preparation of the fluids we used carboxymethyl cellulose with trade-mark "Tylose VHR". The addition Na2CO3in the fluids №2 and №3 was done with the purpose to change the dispersion ability degree of the solid phase particles, which was determined by Figurovsky method.
The researches of the filter cakes received on the fluid loss tester spent on the modified device КТК-2, in which the concave bed and the cylindrical puncheon were replaced, accordingly, with a rough place and metal load with the plane base. It provides preservation of the constant contact area in time between metal and filter cake. Processing of experimental data made on a computer in the program Microsoft Excel: the graphs are plotted and for each period of time the equations in view s = Cs(aut) + Ns и s = ( Ns + Fs(aut) ) are got.
RESULTS AND DISCUSSION
The results of research of the filter cakes frictional properties are submitted in a fig. 2а-4а and in the table 1. The differential curves of the sedimentation analysis demonstrating particle-size distribution of the dispersed phase are submitted in a fig. 2b-4b.
The analysis of the plots Fs(aut) = f (t) (fig. 5) shows, that in first 15minutes the autohesion force is increased, that can be explained by the filter cake compaction under action of load in time and the prevailing in the system of some intermolecular attractive forces. In subsequent 5minutes the autohesion reduction is observed, that can be connected with prevailing intermolecular repulsive forces. In the fluid №1 in 15minutes contact forces of intermolecular interaction, apparently, have the higher values in comparison with the fluids №2 and №3 in all the period of time that is connected to presence of higher number of contacts on unit of an area.
It is interesting the comparison of similar relationships for the fluids №2 and №3 with each other having approximately the identical dispersion ability degree of the solid phase. In 5 and 10minutes contact the value of intermolecular forces working between particles in the fluid №2 is less, than in the fluid №3 (fig. 5, tab. 1). At the same time, the values of frictional factors in the fluid №2 are higher, than in the fluid №3. In 15 and 20minutes the values of forces of intermolecular interaction are more in the fluids №2, but the friction factors have smaller values, than in the fluids №3.
It is necessary to note, that in the fluids №3 the filter cake breaking specific strength constantly decrease in time that specifies prevalence of the repulsive forces. The alternate prevalence of the repulsive and attraction forces in time is marked in the fluid №2. At the same time, observable increase frictional factor in time in the fluid №2 is less intensive, than in the fluid №3.
The executed researches have allowed to appraisal influence of the dispersion ability degree of the solid phase of the clay drilling fluids on frictional properties of the filter cakes. It is established, that the best antifrictional properties have the monodispersed filter cake. Hence, the perspective methods are some methods of homogenization of the dispersed phases of well drilling fluids, alongside with some hardening cakes methods and improvement their properties in a direction of decrease of resistance to shear forces. Realized with our participation a hardening filter cakes method by some chrome content substances for drilling deep wells in Turkmenistan has given positive results for the prevention of sticking.
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